This invention relates to a method of manufacturing a semiconductor device, and more particularly to a method to prevent damage to those parts other than the part to be annealed by selectively heat treating or annealing a silicon semiconductor substrate using irradation of a laser beam.
Where a semiconductor substrate, a silicon substrate for example, is annealed by irradiation of a laser beam, the rate or amount of incident beam of the laser at part of the exposed substrate is different from that at part of the substrate which is covered by an insulating film, such as silicon dioxide (SiO.sub.2). Where laser beam L is irradiated simultaneously, as shown in FIG. 1, onto area A of a silicon semiconductor substrate 1 and area B of substrate 1 which is covered by a film 2 of SiO.sub.2, one might assume that the amount of incident beam of laser L which penetrates into area A is larger than that which enters into area B because area B is covered with the SiO.sub.2 film 2. What happens is quite to the contrary, however and in some instances, the part of substrate 1 in area B is physically damaged. Upon investigation of this, it was determined that SiO.sub.2 film 2 functions as a cover to prevent reflection of the laser beam.
It was thus confirmed that the amount of incident beam of laser L that penetrates into the substrate 1 in area A is always less than that in area B. It was further found that the effectiveness of the film 2 in preventing reflection of laser beam L depends on the thickness of SiO.sub.2 film 2 as well as the wavelength of laser beam L.
Such a relationship is illustrated by a sine wave curve shown in FIG. 2 in which the abscissa represents film thickness t of SiO.sub.2, film 2 expressed relative to the wavelength of laser beam L, and the ordinate represents the rate of incident beam of laser L that penetrates into the substrate 1. In other words, To represents the power (or quantity) of the incident beam of laser L that penetrates into the substrate 1 where there is no film of SiO.sub.2, T represents such power (or quantity) where the substrate 1 is covered with SiO.sub.2 film 2, and the ordinate represents the ratio T/To.
Therefore, where there is no film of SiO.sub.2, that is, where t=0, T/To=1. As will be apparent from FIG. 2, where SiO.sub.2 is formed on the substrate 1, the ratio of penetration, T/To takes a maximum value of approximately 1.3.about.1.4 times that value when t=0. This peak value appears where film thickness t is EQU t=.lambda./4n
or its odd number multiple, where
.lambda. is the wavelength of laser L, and PA1 n is the index of refraction.
For example, with a YAG (yttrium-aluminum-garnet) laser of .lambda.=10600 .ANG., EQU t=.lambda./4n=10600 .ANG./(4.times.1.46).apprxeq.1800 .ANG.
because the index n of refraction of SiO.sub.2 is EQU n=1.46.
That is to say, the first peak value appears where t.apprxeq.1800 .ANG., the second peak appearing at EQU t=3.lambda./4n=5400 .ANG..
Even where t&gt;0, T/To=1 if film thickness t is equal to .lambda./2n or its natural number multiple. In any area other than the one just described, EQU T/To&gt;1
so that the density of energy of incident beam of the laser entering into area B which is covered with SiO.sub.2 film 2 is larger than the energy density in area A as shown by circled A and B in FIG. 2. This is the cause of technical disadvantages as will now be described.
Where the film 2 of SiO.sub.2 shown in FIG. 1 is used as a mask when forming a region 3 by ion implantation and said region 3 is an area A that is desired to be annealed, it cannot be avoided that part of laser beam L is irradiated onto area B when it is irradiated onto the region 3 in order to anneal said region 3 which is in an amorphous state. In such a case, the length of time of laser irradiation and other conditions are established so as to best suit the annealing purpose. If thickness t of SiO.sub.2 film 2 is not .lambda./2n times, a natural multiple number thereof or is not near such a value, there would be excessive irradiation of the laser beam onto area B by the time annealing of area A has been completed. This causes damage to the substrate 1 in area B as was mentioned before.